Most everyone experiences the joy of riding a bicycle at one point in their lifetime, and many learn to ride at a very young age. Because a young child can master its basic principles, the act of riding a bicycle itself appears very simple. The physics behind the exhilarating ride, however, are anything but simplistic. The cyclist needs to overcome numerous types of forces acting on the properties of balancing, steering, braking, accelerating, suspension activation, vibration, and many other bicycling characteristics. Moreover, many of the forces in each physical realm are open to change and depend on their surrounding environment and/or forces from other properties, which adds several orders of complexity.
To consider the complexities of a bicycle as a whole becomes somewhat overwhelming. Separately evaluating each force, however, that acts on the various properties of a bicycle ride makes the task somewhat manageable. For example, if we consider a cyclist or rider and her bicycle as a single system, two groups of forces emerge that act on that system and its components: internal and external forces. Internal forces are mostly caused by the rider and the rider's interaction with the bicycle (e.g., by bicycle component friction). External forces, on the other hand, are due to gravity, inertia, contact with the ground, and contact with the atmosphere.
While the internal forces can have a significant impact on bicycle performance, most any bicycle racer will agree that the largest resistance comes from the induced external force of the bicycle's movement through the air. As a rider attempts to move faster, the atmospheric drag and crosswind forces become greater, which in turn requires the rider to expend greater energy to overcome them. Thus, these forces become an important consideration in bicycle designs, especially in the areas of bicycle racing and triathlons.
Traditionally, bicycle structures such as frames, seat tubes, fork blades, shift levers, etc. have generally circular or otherwise generally uniform smooth curvilinear cross-sectional shapes. Such structures have cross sections with relatively low length-to-width aspect ratios. As used herein, the aspect ratio of a cross section is defined as the unit length over the unit width wherein the length is oriented to be generally aligned with a direction of travel of the bicycle structure. For example, a bicycle structure having a cross section with a circular shape has an aspect ratio of approximately 1. During cycling, bicycle structures having aspect ratios of approximately 1 experience airflow detachment about a portion of the perimeter of the cross section of the bicycle structure. The airflow detachment creates a swirling and often turbulent region of airflow in a wake region generally immediately behind the respective bicycle tube. The wake in the airflow is indicative of energy dissipation and relatively high levels of drag associated with the bicycle structures, and thus, the bicycle.
In an effort to reduce the external drag forces associated with airflow operation of the bicycle, manufactures now design and construct bicycle structures with improved aerodynamic characteristics. One such widely accepted solution has been to provide the bicycle structure in an airfoil shape, which are most often associated with airplane wings, automobile spoilers, marine parts (commonly referred to as hydrofoils or hydrofins), and other aerodynamic systems.
Regardless of the specific application of the airfoil-shaped structure, the cross sections of airfoils generally have lengths that are several times greater than their widths. A forward facing portion of the airfoil, or the leading edge, is generally curved, although other shapes are possible, and configured to be oriented in a forward facing direction relative to an intended direction of travel. Generally, oppositely facing sidewalls extend rearward from the leading edge and converge at a trailing edge of the cross section of the airfoil.
The trailing edge forms the termination of the airfoil and is typically adjacent a narrowed, pointed tail section of the airfoil. A chord that extends between the leading edge and trailing edge of the cross section is indicative of the airfoil length and is generally many times longer than the longest chord extending between the oppositely facing sidewalls of the cross section. Chords that extend between the widest sections of adjacent sidewalls of the airfoil are indicative of the width of the airfoil. Providing an airfoil having a length that is greater than the width yields an airfoil having a cross section with an aspect ratio that is generally many times larger than a value of 1.
The higher aspect ratio allows the airflow directed over the airfoil to conform to the shape of the airfoil and reduces the potential that the airflow will detach from the walls of the bicycle structures (as compared to bicycle structures that have lower aspect ratios or ratios nearer to 1). Similarly, the increased aspect ratio reduces the size of the turbulent wake region that generally forms immediately behind the bicycle structure; thus, reducing the overall external drags of the bicycle or system. Although such airfoil shapes provide reduced drag performance as compared to structures having lower aspect ratios, such shapes are not without their respective drawbacks or limitations.
For example, international bicycle racing regulations limit the permissible cross sections for bicycle frame tubes. These regulations define a maximum length and a minimum width of the shape of the cross section and thereby effectively define a maximum allowable aspect ratio. For many experienced riders, this maximum allowable aspect ratio is far less than ideal for reducing the amount of drag experienced by a rider. That is, many experienced rider's prefer bicycles with enhanced aspect ratios beyond the regulated limits; however, if they wish to engage in many racing events, they must adhere to the imposed limitations. Thus, while airfoil-shaped bicycle structures experience lower levels of drag as compared to traditional blunt cross sections, e.g., circular, the regulated airfoil-shaped tubes cannot realize the aerodynamic improvements possible with airfoils having higher aspect ratios.
In addition to the regulated performance considerations above, practical considerations also limit the attainable aspect ratios of bicycle structures. For example, as the length of the cross section increases and the width of the cross section decreases with increased aspect ratios, the strength and/or lateral stiffness of the bicycle structure decreases. In other words, the elongated shape of the cross section that improves airflow also detracts from the lateral strength of the bicycle structure. Although attempts to resolve this relationship yielded frame assemblies with improved lateral strength performance, they often inherit increased weight that nearly offset the benefits achieved with the improved aerodynamic performance. Accordingly, there exists a fine balance between the structural integrity and the weight of the bicycle frame when altering the shape of the cross section to achieve a desired aspect ratio.
Another shortcoming of many known airfoil constructions is the difficulty associated with forming the tapered tail section of the airfoil shape. The tail of a common airfoil-shaped structure is relatively narrow and gradually transitions to the generally pointed trailing edge of the airfoil. Forming a blemish free pointed tail section is fairly difficult to manufacture and can be particularly problematic in the composite molding processes that are commonly utilized for manufacturing bicycle structures such as frames, frame tubes, fork tubes, and the like. Simply, it is difficult to maintain the desired shape of the frame tube sections with the materials and processes common to current bicycle frame construction.
Accordingly, there exists a need for a bicycle structures with improved aerodynamic performance that do not overly detract from the lateral strength of the system and preferably comply with international bicycle racing regulations.